The present invention relates to a method for the production of micropowders from cellulose ethers or cellulose, by which fine and extremely fine powders, not available by the methods known until now, can be obtained in a simple manner.
Numerous methods are known for comminuting and grinding the various grades of cellulose and cellulose ethers. Coarse shredding can be effected in a cutting mill. This yields normally a woolly, that is, long-fiber, product of low bulk weight. According to another method, a mat or fleece is produced from cellulose ethers by subjecting them to friction in rolls. There results a rolled mat or fleece which is broken and comminuted in impact cutters or pinned disk mills. The disadvantage of this method consists in that, on the one hand, a considerable proportion of long-fiber material always remains which must be screened and recirculated, and that on the other hand only relatively coarse products can be obtained.
Further, methods are known where the moist reaction products obtained in the production of cellulose ethers are kneaded while adding more water, coarsely shredded and then ground in hammer mills. Only thereafter is the moist product dried. By this method, however, practically only granulated materials can be obtained which after further grinding again show a more or less pronounced long-fiber structure.
A further known process is the grinding of cellulose ethers in ball or vibration grinding mills. For the production of fine powders of cellulose ethers of particle sizes of about 90% less than 100.mu., however, numerous passes are necessary in ball as well as in vibration grinding mills, with recirculation of the screen residue. Since with each mill pass a degradation of the macromolecules takes place, the products no longer fulfill the requirement of forming high-viscosity aqueous solutions. With the repeated mill passes necessary for fine powders, the loss of viscosity may be about 70% to 75% of the initial viscosity.
Lastly, these products are very inhomogeneous with respect to the chain length distribution of the macromolecules because of the different number of mill passes.